Oracle 11g Release 2

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create table reviews (product_id, review_date, release_date, reviews_count) as (
    select 1, date '2015-01-20', date '2015-01-25', 0 from dual union all
    select 1, date '2015-01-23', date '2015-01-25', 0 from dual union all
    select 1, date '2015-01-24', date '2015-01-25', 0 from dual union all
    select 1, date '2015-01-29', date '2015-01-25', 0 from dual union all
    select 2, date '2015-01-20', date '2015-01-25', 0 from dual union all
    select 2, date '2015-01-22', date '2015-01-25', 0 from dual );
6 rows affected

merge into reviews a 
using (select product_id, count(product_id) cnt from reviews
        where review_date < release_date 
        group by product_id ) b
on (a.product_id = b.product_id)
when matched then update set reviews_count = b.cnt;
6 rows affected

select * from reviews


PRODUCT_ID
REVIEW_DATE
RELEASE_DATE
REVIEWS_COUNT


1
20-JAN-15
25-JAN-15
3

1
23-JAN-15
25-JAN-15
3

1
24-JAN-15
25-JAN-15
3

1
29-JAN-15
25-JAN-15
3

2
20-JAN-15
25-JAN-15
2

2
22-JAN-15
25-JAN-15
2

PRODUCT_ID	REVIEW_DATE	RELEASE_DATE	REVIEWS_COUNT
1	20-JAN-15	25-JAN-15	3
1	23-JAN-15	25-JAN-15	3
1	24-JAN-15	25-JAN-15	3
1	29-JAN-15	25-JAN-15	3
2	20-JAN-15	25-JAN-15	2
2	22-JAN-15	25-JAN-15	2